Electric Axle

The electric axle is an add-on to a front-wheel drive or rear-wheel drive transmission. There is an electric motor built on the non-driven axle, this way achieving an electric all-wheel drive without mechanical connection between the engine and the second axle. The electric axle can be added theoretically to any kind of base transmissions, but typically it is enhancing a parallel hybrid transmission.

The electric axle can either boost the base transmission, recuperate during braking or drive the vehicle on its own. The complete driveline can operate as a series hybrid, if the engine is charging the battery through the base transmission, while the electric axle drives.

The electric motor is attached to the axle through a transmission, reducing the rotational speed, so that the motor rotates faster than the axle. There are two basic layouts for the electric axle.

The one is a co-axial layout, when the motor and the axle are concentric and the speed reduction is achieved through a simple planetary gearset.

The other is the parallel layout, when the motor shaft and the axle are parallel but with a center distance. In this case the speed reduction is achieved via gearwheel pairs. Typically, one pair of gearwheels is not sufficient to achieve the necessary ratio, therefore a two-stage transmission is applied with an intermediate shaft.

The speed reduction between the motor and the axle is needed to multiply the drive torque on the axle. However, to prevent the motor from overspeeding, the operation of the electric axle is limited to a certain vehicle speed, above which no electric boosting is possible.

In order to keep the scaled-up drive torque at low speeds, but operate at high vehicle speeds as well, two-speed electric axles are becoming more common: the first gear with high ratio for the low vehicle speed range, the second gear with low ratio for the high vehicle speeds. In the most simple axle design, the gears are shifted via a single synchronizer.

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